The present invention relates to a technique of evaluating a processing effect of various processes using laser light. The invention also relates to a technique of relatively evaluating and controlling illumination energy of laser light.
“Low-temperature processes” are now being developed to manufacture a liquid crystal panel using polysilicon thin-film transistors (TFTs). This is intended to suppress the cost of a liquid crystal panel itself by using a low-temperature process, which allows use of a glass substrate with which a large-size substrate can be obtained at a low cost.
To realize a low-temperature process, the key subject is to crystallize an amorphous silicon film formed on a glass substrate by a heating process of less than about 600° C., a temperature range in which the glass substrate can endure. There is known a low-temperature process in which an amorphous silicon film is formed on a glass substrate by CVD and converted to a crystalline silicon film by illumination with excimer laser light. In this process, the amorphous silicon film is crystallized by instantaneously rendering the surface and its vicinity of the amorphous silicon film into a molten state.
A crystalline silicon film that has been crystallized by illumination with laser light, particularly excimer laser light, is advantageous in that it is close and has superior electrical characteristics. Further, a substrate receives very little thermal damage. However, the excimer laser light is associated with a problem that its illumination energy is unstable, resulting in a difficulty in keeping the optimum illuminating condition.
In the excimer laser, a particular gas is excited by subjecting high-frequency discharging to it, and electromagnetic waves are utilized, which are emitted when molecules of the gas transfer from the excited state to the steady state. Therefore, there exists a problem originating from the principle that when laser oscillation is continued, increase of impurities in the gas or change in quality of the gas itself lowers the laser light output even with application of the same discharge power. It is a general procedure to obtain a constant laser light output by using a calibration table or the like. But this is not always satisfactory. (For example, the illumination energy of laser light is greatly varied by contamination or the like in a discharge chamber.)
It has been proved that the characteristics of a thin-film transistor produced by using a crystalline silicon film that has been crystallized by illumination with laser light approximately depend on the illumination energy of laser light. Therefore, if the illumination energy of laser light can be made constant or a desired value, a thin-film transistor having intended characteristics can be obtained. This is not limited to the thin-film transistor, but also is widely applicable to other semiconductor devices that are produced by a process including laser light illumination.
There are several methods of evaluating the annealing effects of laser light illumination on a semiconductor. Examples of these techniques are disclosed in Japanese Unexamined Patent Publication Nos. Sho. 58-15943, Sho. 58-40331, and Hei. 1-16378.
In these methods, prescribed anneal effects of laser light illumination on a semiconductor, particularly its crystallinity, are measured by Raman spectroscopy, to evaluate the annealing effects. However, the Raman spectroscopy has the following problems.
(1) Bad reproducibility of measurements.
(2) Use of a large-output laser such as an Ar laser causes a problem in safety.
(3) An expensive apparatus is needed.
(4) A measurement takes long time.
It is difficult to evaluate the flatness of a film surface by the Raman spectroscopy, through the flatness of a film surface is an important factor of determining the characteristics of a thin-film transistor manufactured. Thus, a crystalline silicon film to be used for a thin-film transistor having desired characteristics cannot be evaluated sufficiently only by the Raman spectroscopy.
In the above circumstances, at present, in addition to the above-described evaluation using the Raman spectroscopy, the flatness of a film is evaluated by human eyes using an optical microscope or a SEM (scanning electron microscope).
As described above, at present, a crystalline silicon film is produced in the following manner, and a thin-film transistor having desired characteristics is formed by using the crystalline silicon film thus produced.
(A) In a process of crystallizing an amorphous silicon film by using excimer laser light, the optimum illumination condition of excimer laser light is found experimentally. And it is tried to always perform laser light illumination under the optimum condition.
(B) The optimum condition is set by evaluating the crystallinity of the film by the Raman spectroscopy and evaluating its flatness by visual observation.
However, as described above, the illumination energy of excimer laser light is liable to vary and it is difficult to control the illumination energy. As mentioned above as item (B), the evaluation of the effects of laser light illumination depends on the two parameters of the crystallinity evaluation by the Raman spectroscopy and the evaluation of the film flatness by visual observation. It is therefore difficult to control, using the two parameters, the illumination power of excimer laser light, which tends to gradually change, so that it is kept at the optimum value.